Miniature microphone assemblies regularly comprise a capacitive microphone transducer electrically coupled to an integrated circuit die that comprises suitable signal amplification and conditioning circuitry. The signal amplification and conditioning circuitry may comprise a low-noise preamplifier or buffer, frequency selective filters, a DC bias voltage generator etc., adapted to amplify/buffer, filter or perform other forms of signal conditioning to weak signals generated by the microphone transducer in response to impinging sound. The integrated circuit die may comprise a die electrical terminal, for example, a signal input signal terminal or a DC bias voltage terminal, electrically coupled to the capacitive microphone transducer. It is highly desirable and advantageous to provide extremely high input impedance at this die electrical terminal to, for example, optimize the noise properties of the miniature microphone assembly. An extremely high input impedance at the signal input terminal ensures that loading of the capacitive microphone transducer is minimized so as to prevent attenuation of weak audio signals generated by capacitive microphone transducer. A capacitive microphone transducer, suitable for use in miniature microphone assemblies, is usually a device with a very high generator impedance, for example, an impedance corresponding to a capacitor with a value between 0.5 pF and 10 pF.
Accordingly, this signal input terminal of the integrated circuit die is customary designed to present an input impedance higher than 100 GΩ, such as higher than 1 TΩ (1012Ω) or even several TΩ. The input impedance is often determined by an independent bias network on the integrated circuit die, for example, a pair of reverse biased diodes, in combination with the previously-mentioned amplification and conditioning circuitry operatively coupled to the signal input terminal or pad.
However, experimental work conducted by the present inventors has demonstrated that it is very difficult to maintain this extremely high input impedance when the assembled miniature microphone is exposed to realistic environmental conditions for example moisture, cyclic heat and/or exposure to polluting agents. Under such adverse conditions, the input impedance of the integrated circuit can be significantly degraded by a formation or absorption of a thin electrically conducting layer of moisture or water on those surfaces of the microphone carrier and/or the integrated circuit die on which the carrier electrical contact and the die electrical terminal are arranged. The formation or absorption of the thin electrically conducting layer of moisture may be caused by condensation or constant high humidity. The effect is the formation of parallel resistive path, or current leakage path, between the signal input terminal or the carrier electrical contact and another electrical contact of the carrier and/or integrated circuit die—for example a ground contact or a DC supply contact. This causes a detrimental reduction of the input impedance at the signal input terminal from the desired range above 100 GΩ down to a range below a few GΩ or even down to the MΩ range. The reduced input impedance causes a significant increase in the noise level of the miniature microphone assembly.
According to the present invention, the above-mentioned problem is solved by encapsulating electrical terminals of the microphone carrier and the integrated circuit die in a cavity.
Miniature microphone assemblies in accordance with the present invention are well-suited for application in a diverse range of portable communication devices such as cellular or mobile phones, hearing aids, PDAs, game consoles, portable computers etc.